stream: remove always-false condition check

Remove comparison to null of variable guaranteed to be a boolean.

PR-URL: nodejs#41488
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

lib/internal/streams/utils.js

@@ -119,14 +119,14 @@ function isReadableFinished(stream, strict) {
 function isReadable(stream) {
   if (stream && stream[kIsReadable] != null) return stream[kIsReadable];
   const r = isReadableNodeStream(stream);
-  if (r === null || typeof stream?.readable !== 'boolean') return null;
+  if (typeof stream?.readable !== 'boolean') return null;
   if (isDestroyed(stream)) return false;
   return r && stream.readable && !isReadableFinished(stream);
 }
 
 function isWritable(stream) {
   const r = isWritableNodeStream(stream);
-  if (r === null || typeof stream?.writable !== 'boolean') return null;
+  if (typeof stream?.writable !== 'boolean') return null;
   if (isDestroyed(stream)) return false;
   return r && stream.writable && !isWritableEnded(stream);
 }